Water quality management is a critical factor in recirculating aquaculture systems (RASs) that significantly influences the growth and health of fish. The present research aimed to investigate the effects of humic acid (HA) added at three concentrations (0, 1.5, and 3 mg L−1) to water on total ammonia nitrogen (TAN) removal efficiency of biofilter unit, growth performance, and some blood biochemical indices of Nile tilapia (Oreochromis niloticus) in a water recirculating system. The Nile tilapia fish with an average weight of 83.44 ± 1.32 g were cultured in nine individual experimental units for 30 days. The removal efficiency of the biofilter for TAN reached to 81.23% and 82.76% in the systems with 1.5 mg L− 1 and 3 mg L−1 HA, respectively. The systems with HA had higher nitrate (NO3 −) concentration than the the system without HA (control) (p < 0.05). HA adding to the water increased the fish final body weight and the specific growth rate (SGR) compared to the control group (p < 0.05). The feed conversion ratio (FCR) in the HA-groups was significantly lower than the control (p < 0.05). The highest crude protein content of muscle tissue was observed in the fish cultured in the system with 3 mg L−1 HA (p < 0.05). There were no significant differences in the plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) concentrations among treatment groups (p > 0.05). The plasma glucose concentration was significantly lower in the fish exposed to HA compared to the control (p < 0.05). The highest plasma triglyceride concentration was recorded in the HA-3 mg L−1 (p < 0.05). These results indicated that the addition of low-cost commercial HA to RASs has a high potential to improve water quality and fish growth performance.

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